System, Apparatus, and Method for Reducing Inrush Current in a Three-Phase Transformer

ABSTRACT

A system for reducing inrush current in a three phase utility transformer upon energization of the transformer by an applied three phase voltage utilizes a pre-flux circuit for establishing residual flux levels in the core segments of the primary windings of the transformer which are near the prospective flux levels established in the core segments by the applied voltage. The pre-flux circuit includes a pre-fluxing capacitor which, after being charged to a predetermined voltage level, is discharged serially through two of the primary windings to establish the predetermined flux levels in the core segments of the two windings, and a reduced flux level in the core segment of the remaining primary winding. The transformer is energized at the instant of positively-referenced peak phase voltage to the third primary winding such that prospective and residual flux approach a near-equal level in all three core segments and inrush current is reduced. The method may be applied to the secondary or tertiary windings instead of the primary windings. Additionally, an alternative method allows application of the pre-fluxing circuit to a delta-connected set of transformer windings.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

BACKGROUND OF THE INVENTION

The present invention relates generally to transformer inrush currentreduction, and more specifically, to a system, apparatus, and method forreducing inrush current when energizing a three-phase transformer.

When energizing a three-phase transformer in an electric power deliverysystem (such as an electric power generation, transmission, ordistribution system, or the like), inrush currents may occur which canbe as large as ten times the transformer's nominal current and can lastfor up to around half of a second. The actual magnitude of these inrushcurrents depends on the impedance of the source supplying thetransformer, the residual magnetic flux existing in the transformer, andthe angle of the applied voltage at the time of energization.

Such high transformer inrush currents have a number of potentiallyadverse effects. For one, a high inrush current can significantly heattransformer windings and cause deterioration of insulation in thetransformer. In addition, high inrush currents can place largemechanical stresses on the transformer windings sufficient to displacethe windings on the transformer core, which, in the worst case, canbreak electrical connections within the transformer. Moreover,insulation compression from displacement of the transformer windings canresult in turn-to-turn faults within the transformer, which, if leftundetected, can eventually destroy the transformer.

Large inrush currents can also disrupt a power system by inappropriatelytripping circuit breakers and over-current relays, by causing voltagesags which can affect sensitive equipment, by introducing large harmoniccomponents, and by instigating sympathetic inrush currents in adjacent,parallel-connected transformers.

Some prior art inrush current reduction methods which have beenpreviously used or suggested include 1) the use of resistors or othercomponents temporarily switched in series or in parallel with thetransformer windings; 2) the use of controlled voltage energizationwithout accounting for the residual flux in the transformer; and 3) theuse of controlled voltage energization based on an estimate of theresidual flux in the transformer windings.

These prior methods have not been entirely satisfactory for use in powerdelivery systems because they have either required the temporaryinterposition of additional components in one of the circuits of thetransformer during energization or have required circuit breakerscapable of individual phase control with the possible need to measureresidual flux levels in the transformer winding, which can befailure-prone or in some cases require significant upgrade of existingequipment.

The present disclosure provides a system, apparatus and method foreffectively performing this pre-fluxing operation in a conventionalthree-phase transformer. In particular, the present disclosure isdirected to a system, apparatus and method for establishing pre-fluxlevels in the three core segments associated with the transformerprimary windings of a three-phase transformer such that when thewindings are energized at the correct instant significantly reducedinrush current results. The system is automatic, requiring only useractuation prior to energization of the transformer.

OBJECTS OF THE INVENTION

Accordingly, it is a general object of the present disclosure to reducetransformer inrush current in a three-phase power system.

It is a more specific object of the present disclosure to provide asystem, apparatus and method for automatically establishing residualflux levels in the transformer's core segments such that the transformercan be energized with reduced inrush current in the windings.

It is a still more specific object of the present disclosure to providea system, apparatus and method for establishing residual flux levels ofnearly-equal magnitude but opposite polarity in two of the transformer'score segments and a reduced flux in the remaining core segment of athree-phase transformer such that the transformer can be energized withreduced inrush current in the windings by energizing all phases of thetransformer at the same instant as defined by a positively referencedvoltage peak on the remaining winding.

BRIEF DESCRIPTION OF THE DRAWINGS

Although the characteristic features of this disclosure will beparticularly pointed out in the claims, the disclosure itself, and themanner in which it may be made and used, may be better understood byreferring to the following description taken in connection with theaccompanying drawings forming a part hereof, wherein like referencenumerals refer to like parts throughout the several views and in which:

FIG. 1 is a simplified perspective view of a power delivery substationincorporating a pre-fluxing system constructed in accordance with thedisclosure.

FIG. 2 is a simplified schematic diagram partially in functional blockform showing a pre-fluxing system constructed in accordance with thedisclosure.

FIG. 3 is a simplified schematic diagram of a portion of the pre-fluxingsystem of FIG. 2.

FIG. 4 is a simplified depiction of a prefluxing circuit coupled to themagnetic core and windings of a conventional three-phase powertransformer.

FIG. 5 is a simplified graphical depiction of the operation of thepre-fluxing system relative to the three primary windings of thetransformer of FIG. 4.

FIG. 6 is a simplified graphical depiction of the application of voltageto the transformer of FIG. 4 after it has been pre-fluxed.

FIG. 7 is a simplified flow diagram illustrating the sequence of theprincipal operations performed by the pre-fluxing system of FIGS. 1-3.

FIG. 8 a is a simplified depiction of a prefluxing circuit coupled tothe magnetic core and windings of a single-core, three-leg powertransformer.

FIG. 8 b is a simplified depiction of a prefluxing circuit coupled tothe magnetic core and windings of a single-core, three-leg powertransformer.

FIG. 8 c is a simplified depiction of a prefluxing circuit coupled tothe magnetic core and windings of a single-core, five-leg powertransformer.

FIG. 8 d is a simplified depiction of a prefluxing circuit coupled tothe magnetic core and windings of a three-core power transformer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

One method proposed by the present inventor for reducing inrush currentin a single-phase transformer involves establishing a residual flux inthe transformer core by means of a pre-fluxing circuit attached to oneof the transformer's windings. The residual flux established in thetransformer core approaches the prospective flux (the flux in thetransformer core under steady state conditions) which will be producedwhen the winding is energized at a specific system voltage angle. As aconsequence, inrush current to the pre-fluxed winding can besignificantly reduced upon energization of the winding. The pre-fluxingdevice establishes the appropriate residual flux in the transformer coreby supplying an appropriate amount of volt-seconds, also known as fluxlinkages, to the transformer core.

The system of the present disclosure reduces inrush current in athree-phase transformer by simultaneously establishing residual fluxlevels in each of the three core segments associated with thetransformer primary windings which levels are near the prospective fluxlevels corresponding to the applied three voltage phases at the instantof energization of the transformer. More specifically, the presentdisclosure provides for pre-fluxing a three-phase transformer byapplying volt-seconds of an appropriate amount serially to two of theprimary windings to produce nearly equal but opposite magnetic residualflux levels near to the prospective flux levels of the core segmentsassociated with the two windings at the time of energization, and areduced flux level in the core segment associated with the third primarywinding. Then, by causing all three phases of the transformer to beenergized at the same instant by a three-phase voltage source of whichthis instant is defined when the third (non-prefluxed) primary windingis at a “positively-referenced” voltage peak (as explained furtherherein), referenced to the “positively” fluxed core, and by havingestablished the residual flux levels in the two pre-fluxed core segmentsnear the then existing prospective flux levels of the associated phasesat the instant of energization, inrush current is effectively reduced.

Referring to the drawings, and particularly to FIGS. 1 and 2, a powerutility substation 10 is seen to include a three-phase transformer 11 ofconventional design having high voltage primary terminals 12, 13 and 14corresponding to the A, B and C phases of the transformer, respectively,and low voltage secondary output terminals 15, 16 and 17 (FIG. 2)connected to a load (not shown) by a conductor 18. Within transformer 11three Y-connected primary windings 19, 20, and 21 (FIG. 2) are eachconnected at one end to the transformer input terminals 12, 13 and 14,respectively, and at their other end to a common ground 22. Threedelta-connected secondary or tertiary windings 23, 24 and 25 areconnected to output terminals 15, 16 and 17 of the transformer.

Power is supplied to primary windings 19, 20 and 21 by a three-phasesupply line consisting of conductors 26, 27 and 28 connected off-site toa conventional three-phase generator 29 (FIG. 2). A three-gang breaker30 having contacts 31, 32 and 33 serially connected in conductors 26, 27and 28 between generator 29 and the primary terminals 12, 13 and 14 oftransformer 11 enables the three-phase connection to transformer 11 tobe selectively interrupted or established in response to a controlsignal applied to an actuator solenoid 34 associated with the breaker.Another three-gang breaker 35 having respective contacts in conductors36, 37 and 38, and having an actuator solenoid 39 (FIG. 2) enables thetransformer secondary windings 23, 24 and 25 to be disconnected from thelow-voltage transformer load circuit 18.

To reduce inrush current to transformer 11 upon closure of breaker 30,substation 10 includes, in accordance with the disclosure, a pre-fluxsystem 40. As shown in FIGS. 2 and 3, this system includes a dielectricpre-fluxing capacitor 41 which is charged to a pre-determined voltagelevel by a direct current power supply 42 through a pair ofcharge-current limiting resistors 43 and 44 and the normally opencontacts of a pre-charging control breaker 45. Breaker 45, which may beconventional in design and operation, is actuated by a solenoid 46. Toestablish a pre-determined residual flux in the core segments associatedwith the three primary windings 19, 20 and 21, capacitor 41 is seriallydischarged through the B and C phase primary windings 20 and 21 througha circuit which includes conductors 47 and 48 and the normally opencontacts of a pre-flux breaker 49. Breaker 49, which may be conventionalin design and operation, is actuated by a solenoid 50. A set of contacts51 mechanically connected to the breaker provides an indication ofbreaker status. An isolation breaker 52, having normally-open contactsserially connected in conductors 47 and 48, is provided to isolatepreflux circuit 40 from high voltage lines 27 and 28 when the prefluxcircuit is not in use. Breaker 52, which may be conventional in designand operation, is actuated by a solenoid 53. A series-connected fuse 54opens in the event of excessive current in the charging circuit. A diode55 connected across capacitor 41 together with the capacitance ofcapacitor 41 and the inductance series connected primary windings 20 and21, establish a ¼ cycle resonant discharge circuit between the capacitorand the windings, such that upon closure of the contacts of breaker 49(isolation breaker 52 having been previously closed) the capacitor israpidly discharged through the two serially connected primary windings20 and 21 to establish a residual flux in the transformer magnetic corematerial of a magnitude dependent on the magnetic characteristics of thetransformer core material.

In operation, transformer 11 is first isolated from its supply(generator 29) by activation of solenoid 34 to open contacts 31, 32 and33 of line disconnect breaker 30. The transformer is also isolated fromits load by activation of solenoid 39 to open the contacts ofload-disconnect breaker 35. Breaker 30 preferably includes a set ofmechanically-linked contacts 57 which confirm that the breaker hasopened, and load disconnect breaker 35 preferably includes similarmechanically-linked contacts 58 which confirm that the load disconnectbreaker has operated. Next, pre-flux circuit 40 is connected to primarywindings 20 and 21 by activation of solenoid 53, which closes thecontacts of isolation breaker 52. A set of mechanically-coupled contacts59 mechanically coupled to the breaker provide confirmation that thebreaker contacts have closed.

In preparation for the pre-fluxing operation, capacitor 41 is charged toa predetermined voltage by application of a control signal to solenoid46, which causes closure of capacitor pre-charge breaker 45 to connectthe capacitor to DC supply 42 through charge rate limiting resistors 43and 44. After capacitor 41 has fully charged, breaker 45 is opened andcapacitor 41 remains in its predetermined voltage level. Next, providedline breaker 30 and load disconnect breaker 35 are open, and pre-fluxisolation breaker 52 is closed, pre-flux capacitor 41 is dischargedthrough the serially connected transformer primary windings 21 and 22 bymomentary activation of solenoid 50 to close pre-flux breaker 49. Aftercapacitor 41 has fully discharged through windings 21 and 22, breaker 49is opened.

Since the predetermined desired pre-flux state has at this time beenestablished in the two primary windings, isolation breaker 52 is openedto disconnect pre-flux circuit 40 from supply lines 27 and 28 and theprimary windings of transformer 11. As shown in FIG. 1, the function ofisolation breaker 52 may in practice be advantageously accomplished bytwo separate identical breakers having their actuation solenoidsconnected in parallel and contacts serially connected in respective onesof prefluxing conductors 47 and 48.

As generally shown in FIG. 4, a generator 29 (see FIG. 2) is coupled totransformer 11 (see FIG. 2) through contact 30, which is controlled bybreaker controller 81 as described herein. Transformer 11, in accordancewith conventional practice, includes a magnetic core 60 having coresegments 61, 62 and 63 associated with primary windings 19, 20 and 21,and secondary windings 23, 24 and 25, respectively. The discharge ofcapacitor 41 through windings 20 and 21 causes approximately ½ of thecapacitor 41 voltage to appear across each of windings 20 and 21, whichestablishes a residual flux in the associated core segments 62 and 63.Moreover, by reason of the purposeful connection of the capacitor 41 tothe transformer windings 20 and 21 in such a manner that the directionof the flux developed is considered “positive” in core segment 63, and“negative” in core segment 62. In the case of an ABC phase rotation andusing B & C phase windings 20 & 21, capacitor's 41 positive voltageterminal XX is connected to the polar (dotted) terminal 14 of the Cphase winding 21 and the capacitor's 41 negative terminal XY isconnected to the polar (dotted) terminal 13 of the B phase winding 20.The flux in core segments 62 & 63 generates a voltage in thedelta-windings of nearly equal magnitude but opposite polarity. Becauseof the delta winding and Kirchhoff's Voltage Law, the voltage in thedelta winding of the uninvolved core segment 61 has a voltage whichtends towards zero developed across it which encourages a reducedresidual flux (which may be a near-zero flux) in core segment 61.

The status of the residual flux in the three windings followingdischarge of capacitor 41 is illustrated in FIG. 5. There it is seenthat a residual flux 64 in the core segment 63 of the phase C primarywinding 21 exists in a relatively positive direction as a consequence ofthe voltage 65 supplied from capacitor 41 being in a relatively positivedirection, and that a residual flux 66 in the core segment 62 of thephase B primary winding 20 exists of nearly the same magnitude asresidual flux 64 but in a relatively negative direction as a consequenceof the voltage 67 supplied from capacitor 41 being in a relativelynegative direction. Moreover, it is seen that the opposite directionfluxes in core segments 62 and 63, through the use of thedelta-connected winding, encourage a near-zero residual flux 68 in thecore segment 61 of the A phase primary winding 19 (See FIG. 4).

In accordance with the disclosure, and as illustrated in FIG. 6, theabove-described prefluxing of primary windings 20 and 21 enables theinrush current to transformer 11 to be reduced by controlling theclosure of line breaker 30 such that the voltage applied to the A phaseprimary winding 19 is at its positively-referenced peak at the instantof energization (i.e. at the closing of breaker 30). Since the A phaseprospective flux, which lags the A phase voltage 71 by 90°, is at thisinstant zero, the prospective flux is near the residual flux, and inrushcurrent is reduced.

At the same instant of energization, the B phase system voltage 72,which in a three-phase system lags the A phase system voltage by 120°,is at a predetermined relatively negative level 73 and the correspondingB phase prospective flux 74 is at a predetermined relatively negativelevel 75, which, approaches the pre-established phase B residual flux 66(FIG. 5). Similarly, at the same instant of energization, the C phasesystem voltage 76, which lags the B phase system voltage by 120°, is ata predetermined relatively positive level 77 and the corresponding Cphase prospective flux 78 is at a relatively positive level 79, whichapproaches the previously established C phase residual flux 64 (FIG. 5).

The operation of line breaker 30, load breaker 35, isolation breaker 52,pre-flux breaker 49 and capacitor pre-charge breaker 45 are controlledby a pre-flux system 40. In one embodiment, such operation may becontrolled by the breaker controller 81, and in one embodiment, suchoperation may be controlled by the pre-flux system controller 80, and inone embodiment, such operation may be controlled by a combination of thebreaker controller 81 and the pre-flux system controller 80. Asillustrated by the simplified flow chart of FIG. 7, this controller,upon receiving a user-initiated start command at 101, functions at 103to disconnect the three-phase transformer 11 from its power source andload by actuating solenoids 34 and 39 to open breakers 30 and 35,respectively. Next, after the opening of these breakers has beenconfirmed by respective mechanically coupled contacts 57 and 58, thecontroller charges the pre-fluxing capacitor 41 to a predeterminedvoltage 105 level by actuating solenoid 46 to close breaker 45. Afterthe capacitor has been fully charged, breaker 45 is opened and in 107the controller connects the pre-fluxing system 40 to transformer B-phaseand C-phase primary windings 20 and 21 by actuating solenoid 53 to closeisolation breaker 52. After the closure of breaker 52 has been confirmedby contacts 59, solenoid 50 is actuated at 109 to close pre-flux switch(breaker 49) to discharge capacitor 41 to create a ¼-cycle resonancebetween capacitor and transformer B-phase and C-phase primary windings.Then, after the capacitor has discharged, and breaker 49 has opened, asconfirmed by contacts 51, the controller disconnects the pre-fluxingsystem 40 from the transformer 111 and the high voltage supplyconductors 27 and 28 by opening isolation contactor 52. It remains forcontroller 80 to now at 113 simultaneously energize transformer windingsfrom a three-phase source at an “A”-phase positive voltage peak. Thecontroller accordingly closes breaker 30 at the positively-referencedvoltage peak of the A phase conductor 26. By reason of the residual fluxlevels of the core segments 61, 62 and 63 of windings 19, 20, and 21closely matching the A, B and C phase prospective flux levels at theinstant of closure, respectively, inrush current to transformer 11 isreduced.

For the system to be effective it is necessary that breaker 30 connectall three phases of the high voltage supply line to the primary windingsof transformer 11 simultaneously upon occurrence of apositively-referenced voltage peak on the zero residual flux phase. Thispositively referenced voltage peak is the voltage peak causing the polar(dotted) terminal of the zero residual flux phase to be at a positivepotential with respect to the ground reference (shown in FIG. 6). Tothis end, a three phase quick-connect breaker having a high degree ofrepeatability is utilized in combination with a breaker controller 81.Voltage on the zero residual flux phase is sensed by a conventionalsensor 82 to develop a voltage phase sense signal which is supplied tothe breaker controller 81. Sensor 82 needs to be wired to controller 81such that it has the same positive reference as the transformer winding(see FIG. 4). Breaker controller 81, upon receipt of a command signalfrom system controller 80, senses the timing of a voltage minimum onconductor 26 and, taking into account the characteristics of thebreaker, sends a command signal to controller 80 to cause the actuatorsolenoid 34 of breaker 30 to be timely actuated to close the breaker atthe desired instant, in this case the positive voltage peak. Breakercontroller 81 may for example, be an appropriately programmed model SEL352 Relay manufactured by Schweitzer Engineering Laboratories, Inc., ofPullman, Wash.

In operation, when a three-phase sinusoidal voltage is applied to theprimary windings of the transformer an accompanying prospective flux isgenerated in the magnetic core segments associated with the windingswhich lags the applied voltage by 90°. The magnitude of the inrushcurrent in each winding diminishes as the magnitude of the prospectiveflux approaches the magnitude of the residual flux in the winding, andapproaches zero when the fluxes are equal. To this end, the presentdisclosure establishes a residual flux level in each core segment whichapproaches the prospective flux level that is generated in each coresegment by the applied three-phase voltage.

By establishing a near-zero flux level in the core segment associatedwith one of the three primary windings, and equal but opposite magneticpolarity flux levels in the two remaining core segments associated withthe remaining two primary windings, it is only necessary that thetransformer primary windings be energized at the same instant that thephase voltage applied to the winding associated with the near-zero fluxcore segment is at a positively-referenced voltage peak. The inducedvoltages occurring in the delta-connected secondary windings of thetransformer assisted in establishing the near-zero residual flux in thecore segment during the pre-fluxing operation.

The desired residual flux levels in the three core segments are achievedby discharging capacitor 41, which has been pre-charged to apredetermined voltage (i.e. charge level), serially through the twoprimary windings to establish the necessary residual flux levels in thecore segments associated with the two windings.

The capacitance of the pre-fluxing capacitor is selected to resonatewith the magnetizing inductance presented by the two series-connectedprimary windings, which causes the capacitor to be fully dischargedwithin one-quarter cycle of the resulting resonate frequency. After thecapacitor discharges its energy into the transformer, the diode placedacross the capacitors terminals begins conducting to prevent reversecharging of the capacitor. The capacitor is essentially shorted at thispoint and the current decays to zero according to the L/R time constantat which point the transformer residual flux is established.

While pre-fluxing of the B and C phase windings with ABC phase rotationhas been shown in the embodiment of FIGS. 1-6, it will be appreciatedthat any two primary windings can be prefluxed for either ABC or ACBphase rotation, provided that the transformer is energized at thepositively-referenced voltage peak of the phase of the remainingnear-zero flux winding, and that the polarity of the voltage supplied bythe pre-fluxing capacitor will be in accordance with the direction ofphase rotation of the applied three phase voltage and the selectedpositively-referenced voltage peak. The following rule holds for anyphase rotation and winding selection: Whichever phase (A, B, or C)positive voltage peak is selected for transformer energization, thephase which precedes it in the phase sequence is the phase that must bepositively fluxed. In the case of energizing at an A phase positivevoltage peak for ABC phase rotation, the C phase must be fluxed with apositive polarity and B phase results with a negative polarity flux. Inthe case of energizing at a C phase positive voltage peak for ACB phaserotation, the A phase must be fluxed with a positive polarity and Bphase results with a negative polarity flux.

The capacitance and initial voltage level of the pre-fluxing capacitoris selected in such a way that during the one-quarter resonance cyclecreated between the pre-fluxing capacitor and the two series-connectedprimary windings a sufficient number of volt-seconds are supplied to thetransformer iron to achieve maximal residual flux in each of the twocore segments of the transformer associated with the pre-fluxed primarywindings.

The following illustrates the calculation of capacitor size for atypical 60 hertz 200 MVA 230 kV three phase utility transformerinstalled in a substation having a 230 volt DC source for charging thecapacitor of the pre-fluxing device:

S_(3 ⌀) = 200  MVA  V_(LL_(P)) = 230  kV  f = 60  Hz$S_{1\; \varnothing} = {\frac{S_{3\; \varnothing}}{3} = {66.67\mspace{14mu} {MVA}}}$$V_{{LN}_{P}} = {\frac{V_{{LL}_{P}}}{\sqrt{3}} = {132.791\mspace{14mu} {kV}}}$V_(cap) = 230  V

Where S_(3) is three-phase apparent power; S_(1) single-phase apparentpower, f is the line frequency; V_(LL) _(—) _(P) is the primary line toline voltage; V_(LN) _(—) _(P) is the primary line to neutral voltage,and V_(cap) is the voltage of the pre-flux capacitor.

Given this development, the capacitance of the pre-flux capacitor can beexpressed as follows:

$C_{pf} = {{( \frac{1}{L_{m_{{approx}_{P}}}} )( {\frac{\sqrt{2}}{2 \cdot \pi} \cdot \frac{V_{{LN}_{P}}}{f \cdot V_{cap}}} )^{2}} = {334.289\mspace{14mu} {mF}}}$

Where C_(pf) is the capacitance of the pre-fluxing capacitor, L_(m)_(approxP) is the approximate magnetizing inductance of the transformerto be prefluxed referred to the transformer's primary side.

And the energy stored in the pre-flux capacitor can be expressed asfollows:

E _(pf)=½C _(pf) ·V _(cap) ²=8.842 kJ

Allowing for losses, E_(pf) can be set equal to 10 kJ. Accordingly,given a typical 100 watt 230 volt DC power supply, the pre-fluxingsystem will require the following charging time for each application:

P_(supply) = 100  W$t_{charge} = {\frac{E_{pf}}{P_{supply}} = {100\mspace{14mu} s}}$

The foregoing description has explained the operation of the disclosedtransformer pre-fluxing system with respect to one type of transformer.However, the disclosed system will work with many different types oftransformer configurations. For example, FIG. 8 a depicts a transformerhaving a single core 60 a with three legs 61 a, 62 a, and 63 a, andthree separate sets of windings. The first set of windings 19 a, 20 a,and 21 a are wye configured, with a generator coupled to winding 19 athrough a circuit breaker 30 that is controlled by breaker-controller81, which receives line data through sensor 82; and a pre-flux capacitordisposed across windings 20 a and 21 a. The remaining windings could beconfigured in delta or wye configurations. Note that transformer of FIG.8 a is depicted without a delta winding, which is acceptable because ina single-core, three-leg configuration, the sum of the flux on the legsapproaches zero, and, because the pre-fluxing device forced the flux intwo of the three legs to substantially equal and opposite values, theflux on the remaining of the three legs should approach zero. In asingle-core, three-leg design (such as the transformer illustrated inFIG. 4), a delta winding may help further force the flux in theremaining of the three legs to approach zero. However, the delta windingis needed for certain other transformer configurations due to thetransformer including more than three legs. FIG. 8 b depicts atransformer having a single core 60 b with three legs 61 b, 62 b, and 63b, and three sets of windings. The first set of windings 19 b, 20 b, and21 b are wye configured, with a generator coupled to winding 19 bthrough a circuit breaker 30 that is controlled by breaker-controller81, which receives line data through sensor 82; and a pre-fluxingcapacitor 41 disposed across windings 20 b and 21 b. The secondarywindings 23 b, 24 b, and 25 b are delta configured, and the tertiarywindings may be either wye or delta configured. FIG. 8 c depicts atransformer having a single core 60 c and five legs, although only legs61 c, 62 c, and 63 c are depicted as having windings. The primarywindings 19 c, 20 c, and 21 c are wye configured, with a generatorcoupled to winding 19 c through a circuit breaker 30 that is controlledby breaker-controller 81, which receives line data through sensor 82;and a pre-fluxing capacitor 41 disposed across windings 20 c and 21 c.The secondary windings 23 c, 24 c, and 25 c are delta configured, andthe tertiary windings may be either wye or delta configured. Inaddition, while all embodiments of the disclosed pre-fluxing systemdisclosed thus far have been discussed in conjunction with a single coretransformer, there is no such limitation in the use of the disclosedsystem. By way of example, FIG. 8 d depicts a transformer having threeseparate cores 61 d, 62 d, and 63 d. The primary windings 19 d, 20 d,and 21 d of the three cores 61 d, 62 d, and 63 d are wye configured,with a generator coupled to winding 19 d through a circuit breaker 30that is controlled by breaker-controller 81, which receives line datathrough sensor 82; and a pre-fluxing capacitor 41 disposed acrosswindings 20 d and 21 d. Secondary windings 23 d, 24 d, and 25 d aredelta configured.

While an embodiment of the disclosure is shown permanently installed ina utility substation, it will be appreciated that the disclosure can bepracticed as a temporarily installed system. In this case thepre-fluxing system 40 would be temporarily connected to the transformerand actuated only after the transformer primary and secondary circuitshad been separately disconnected, as the “load” side of the transformercan, in some cases, energize the transformer. The pre-flux system 40would then be disconnected and the line circuit would then besubsequently connected to the transformer primary winding at apositively-referenced voltage peak to the zero flux primary winding coresegment as previously described to reduce inrush current.

Furthermore, while an embodiment of the disclosure is shown as applyinga pre-flux to the transformer core from the primary windings of thetransformer, it will be appreciated that the disclosure can be practicedas applying the pre-flux to the secondary windings of the transformerusing the principles discussed herein. Indeed, the disclosure can bepracticed using any of the various standard transformer windings usefulfor introducing a pre-flux to the core or cores of the transformer. Someof the various transformer windings useful for introducing a pre-flux tothe core or cores may include the primary windings, secondary windings,or tertiary windings.

Furthermore, while an embodiment of the disclosure is shown as applyinga pre-flux to the transformer core, it will be appreciated that thedisclosure can be practiced on a transformer of the type shown in FIG. 8c or 8 d but that doesn't have a delta-connected secondary ordelta-connected tertiary winding. In this case, the same procedure forpre-fluxing is followed depending on the phase that is selected to beenergized at its positive voltage peak and the phase rotation of thesystem, as was described previously.

Furthermore, while an embodiment of the disclosure is shown as applyinga pre-flux to the transformer core from a set of transformer windingsconnected in a wye configuration, it will be appreciated that thedisclosure can be practiced on a set of transformer windings that areconnected in delta. In this case, the same procedure for pre-fluxing isfollowed depending on the phase that is selected to be energized at itspositive voltage peak and the phase rotation of the system, as wasdescribed previously. That is, the pre-fluxing circuit is used toconnect the pre-fluxing capacitor to the delta windings to discharge thepre-fluxing capacitor through the parallel combination of the firstwinding in one branch and the series combination of the remaining twowindings in the second branch, thus establishing residual fluxes in thecore segment associated with the first winding at a first polarity andmagnitude, and in the remaining two core segments associated with theremaining two windings in an opposite polarity and around half of themagnitude of the residual flux in the one core segment. In applying thepre-fluxing capacitor in this configuration, it is important to note thespecific type of delta connection used by the transformer.

FIG. 9 illustrates a transformer with “DAC” connections that has ABCphase rotation. In this example, the pre-fluxing algorithm may be suitedfor closing on the A phase voltage peak. The example illustrated in FIG.9 includes a transformer core 960 having three cores segments 961, 962,963 each associated with an electrical phase (A, B, C). The transformerincludes a delta winding that includes windings 923, 924, 925 eachassociated with an electrical phase and core segment 961, 962, 963. Thetransformer further includes a second set of windings (which may be theprimary windings to be connected to a generator via breaker 30) in Wyeconfiguration 919, 920, 921 each associated with a core segment 961,962, 963 corresponding to a specific phase. In this example, thetransformer is wound in “DAC” configuration. In this example, followingthe discussion previously about the phase selected to close in on thevoltage peak and the given phase rotation, C phase is the phaserequiring the positive flux. When a delta winding set is connected in a“DAC” configuration, it specifies that the polar (dotted) winding end ofA phase 923 is connected to the non-polar winding end of C phase 925.Thus for C phase core segment 963 to achieve the maximum possiblepositive flux, the pre-fluxing capacitor 41 positive polarity terminal,XX, is connected to the C phase polar (dotted) terminal, YY, and thepre-fluxing capacitor 41 negative polarity terminal, XY, is connected tothe A phase polar (dotted) terminal, YZ, which establishes a positivepolarity flux of a certain magnitude in C phase core segment 963 and anegative polarity flux with around half the magnitude in A and B phasecore segments 961, 962.

Another possible delta winding configuration is a “DAB” which specifiesthe polar (dotted) winding end of A phase windings 923 may be connectedto the non-polar winding end of B phase windings 924 (not as illustratedin FIG. 9). In this example, the polar (dotted) winding end of B phasewindings 924 may be connected to the non-polar winding end of C phasewindings 925 and the polar (dotted) winding end of C phase windings 925may be connected to the non-polar winding end of A phase windings 923.Thus, in the case of a system with ACB phase rotation, and if C phasewere the phase selected to close at the voltage peak, the A phase coresegment 961 would be fluxed with the positive flux. In the case of a“DAB” connected delta winding, the pre-fluxing capacitor 41 positivepolarity terminal, XX, may be connected to the polar (dotted) windingend of A phase windings 923 and the pre-fluxing capacitor 41 negativepolarity terminal, XY, may be connected to the polar (dotted) windingend of C phase windings 925.

The foregoing description of the disclosure has been presented forpurposes of illustration and description, and is not intended to beexhaustive or to limit the disclosure to the precise form disclosed. Thedescription was selected to best explain the principles of thedisclosure and practical application of these principles to enableothers skilled in the art to best utilize the disclosure in variousembodiments and various modifications as are suited to the particularuse contemplated. It is intended that the scope of the invention not belimited by the specification, but be defined by the claims set forthbelow.

What is claimed is:
 1. A pre-fluxing system for a three-phasetransformer to reduce inrush current in the three-phase transformerhaving first A, B and C phase windings, second A, B and C phasewindings, and a magnetic core having A, B and C segments associated withthe first and second A, B and C phase windings, respectively,comprising: a three-phase line circuit including a three-phase linebreaker for applying line voltage to the three-phase transformer; apre-fluxing capacitor; a direct current source for charging thepre-fluxing capacitor; a charging circuit for connecting the directcurrent source to the pre-fluxing capacitor and charging the pre-fluxingcapacitor to a predetermined voltage level; a pre-fluxing circuit forconnecting the pre-fluxing capacitor to two first windings of the firstA, B, and C phase windings; and a control circuit for controlling aclosing of the three-phase line circuit to apply line voltage to thethree-phase transformer coincident with a positively-referenced voltagepeak of a phase associated with a pre-determined winding.
 2. Thepre-fluxing system as defined in claim 1, wherein the first A, B, and Cphase windings are in wye configuration, and the pre-fluxing circuit isconfigured to discharge the pre-fluxing capacitor through the two firstwindings in series to establish residual fluxes of nearly equalmagnitude but opposite polarity in two of the magnetic core A, B, and Csegments associated with the two first windings.
 3. The pre-fluxingsystem as defined in claim 1, wherein the magnetic core comprises threemagnetic cores and wherein at least one of the first and second A, B,and C phase windings are in delta configuration.
 4. The pre-fluxingsystem as defined in claim 1, wherein: the first A, B, and C phasewindings are in delta configuration; and, the pre-fluxing circuit isconfigured to discharge the pre-fluxing capacitor through the first A,B, and C phase windings to establish a first residual flux comprising amagnitude and a polarity in the one of the first A, B, and C segmentsand second residual fluxes in two of the first A, B, and C segments,where the second residual fluxes comprise magnitudes less than that ofthe first residual flux and polarities opposite that of the firstresidual flux..
 5. The pre-fluxing system as defined in claim 1, whereinthe three-phase line circuit comprises a primary line circuit, the firstA, B, and C phase windings comprise primary windings, and the controlcircuit controls a closing of the three-phase line circuit to apply linevoltage to the first A, B, and C phase windings.
 6. The pre-fluxingsystem as defined in claim 1, wherein the three-phase line circuitcomprises a primary line circuit, the first A, B, and C phase windingscomprise secondary windings, and the control circuit controls a closingof the three-phase circuit to apply line voltage to the second A, B, andC phase windings.
 7. The pre-fluxing system as defined in claim 1,wherein the first A, B, and C phase windings comprise one selected fromthe group consisting of: a primary winding, a secondary winding, and atertiary winding.
 8. A pre-fluxing system as defined in claim 1 whereinthe line breaker is closed when the prospective fluxes approach theresidual fluxes of two of the magnetic core A, B, and C segments.
 9. Apre-fluxing system for a three-phase transformer to reduce inrushcurrent in the three-phase transformer having first A, B, and C phasewindings in wye configuration, second A, B, and C phase windings, and amagnetic core having A, B, and C segments associated with the first andsecond A, B, and C phase windings, respectively, comprising: athree-phase line circuit including a three-phase line breaker forapplying line voltage to the three-phase transformer; a pre-fluxingcapacitor; a direct current source for charging the pre-fluxingcapacitor; a charging circuit for connecting the direct current sourceto the pre-fluxing capacitor and charging the pre-fluxing capacitor to apredetermined voltage level; a pre-fluxing circuit for discharging thepre-fluxing capacitor through two first windings of the first A, B, andC phase windings; and, a control circuit for controlling a closing ofthe three-phase line circuit to apply line voltage to the three-phasetransformer coincident with a positively-referenced voltage peak of aphase associated with a remaining winding of the first A, B, and C phasewindings.
 10. The pre-fluxing system of claim 9, wherein the pre-fluxingcircuit discharges the pre-fluxing capacitor to establish residualfluxes of nearly equal magnitude and opposite polarity in two of themagnetic core A, B, and C segments associated with the two firstwindings.
 11. The pre-fluxing system of claim 10, wherein thepre-fluxing circuit discharges the pre-fluxing capacitor to establish areduced flux in a remaining magnetic core segment.
 12. The pre-fluxingsystem as defined in claim 9, wherein the three-phase line circuitcomprises a primary line circuit, the first A, B, and C phase windingscomprise primary windings, and the control circuit controls a closing ofthe three-phase line circuit to apply line voltage to the first A, B,and C phase windings.
 13. The pre-fluxing system as defined in claim 9,wherein the three-phase line circuit comprises a primary line circuit,the first A, B, and C phase windings comprise secondary windings, andthe control circuit controls a closing of the three-phase circuit toapply line voltage to the second A, B, and C phase windings.
 14. Thepre-fluxing system as defined in claim 9, wherein the first A, B, and Cphase windings comprise one selected from the group consisting of: aprimary winding, a secondary winding, and a tertiary winding.
 15. Apre-fluxing system as defined in claim 9 wherein the line breaker isclosed when the prospective fluxes approach the residual fluxes of twoof the magnetic core A, B, and C segments.
 16. A pre-fluxing system fora three-phase transformer to reduce inrush current in the three-phasetransformer having first A, B, and C phase windings in deltaconfiguration, second A, B, and C phase windings, and a magnetic corehaving A, B, and C segments associated with the first and second A, B,and C phase windings, respectively, comprising: a three-phase linecircuit including a three-phase line breaker for applying line voltageto the three-phase transformer; a pre-fluxing capacitor; a directcurrent source for charging the pre-fluxing capacitor; a chargingcircuit for connecting the direct current source to the pre-fluxingcapacitor and charging the pre-fluxing capacitor to a predeterminedvoltage level; a pre-fluxing circuit for discharging the pre-fluxingcapacitor through the first A, B, and C phase windings to establish aresidual flux in each of the A, B, and C segments; and, a controlcircuit for controlling a closing of the three-phase line circuit toapply line voltage to the three-phase transformer coincident with apositively-referenced voltage peak of a phase associated with a selectedsegment of the A, B, and C segments.
 17. The pre-fluxing system of claim16, wherein the pre-fluxing circuit is in connection with two windingsof the first A, B, and C phase windings, for discharging the pre-fluxingcapacitor through a parallel combination of one of the first A, B, and Cphase windings in a first branch, and the remaining two of the first A,B, and C phase windings in series in a second branch.
 18. Thepre-fluxing system of claim 17, wherein the first core segment isassociated with the one of the first A, B, and C phase windings in thefirst branch.
 19. The pre-fluxing system of claim 16, wherein thepre-fluxing circuit is further for discharging the pre-fluxing capacitorto establish a residual flux in a second core segment and a third coresegment of the A, B, and C segments, with polarity opposite a polarityof the residual flux in the first core segment and magnitude less than amagnitude of the residual flux in the first core segment.
 20. Thepre-fluxing system as defined in claim 16, wherein the three-phase linecircuit comprises a primary line circuit, the first A, B, and C phasewindings comprise primary windings, and the control circuit controls aclosing of the three-phase line circuit to apply line voltage to thefirst A, B, and C phase windings.
 21. The pre-fluxing system as definedin claim 16, wherein the three-phase line circuit comprises a primaryline circuit, the first A, B, and C phase windings comprise secondarywindings, and the control circuit controls a closing of the three-phasecircuit to apply line voltage to the second A, B, and C phase windings.22. The pre-fluxing system as defined in claim 16, wherein the first A,B, and C phase windings comprise one selected from the group consistingof: a primary winding, a secondary winding, and a tertiary winding.